The present invention relates to an ignition system for an internal-combustion engine, in particular for use in a chain saw or the like, comprising a magnetically conducting core supporting a charging winding and a triggering winding, a flywheel having at least one magnetic field generating member and adapted to cooperate with the magnetically conducting core to cause, when passing, voltages to be induced in the windings, and an electronic switch means adapted to trigger, in response to a trigger pulse generated by the triggering winding, the discharge of a capacitor, which has been charged by a voltage generated in the charging winding, via the primary of an ignition coil the secondary of which comprises a spark plug.
Internal combustion engines for chain saws and the like often operate at very high speeds, for example up to 14,000 rpm. This means that the ignition system of the engine has to be designed so as to have a large ignition advance in order for the engine to operate properly. At the same time the ignition advance must not be too large when the engine is to be started-up. If the ignition advance is too large at start it can easily happen that reverse motion takes place which may cause injury to the person handling, for instance, a chain saw.
Moreover, internal combustion engines of today are designed to have a high power output which often means high compression in the motor. When such a motor is to be started by pulling a start strap or the like the high compression will have a braking influence on the movement of the piston and the starting course becomes restless. In addition to backstroke it can happen that the engine starts rotating in the reverse direction which is inacceptable and has to be prevented. This course may also take place when the chain saw passes over from a condition of no-load to a condition of heavy load with the accompanying strong drop in engine speed.
One solution to the problem is to design the engine with a particular extended range of ignition advance permitting the required large ignition advance at operating speed and at the same time offering the possibility of late ignition at start where the ignition spark shall appear at a moment corresponding to one or a few degrees before the upper dead center. Such a solution is described in PCT/SE94/01152.
Another way of ensuring that the engine rotates in the correct direction is described in U.S. Pat. No. 5,050,553. In this publication there is described a microprocessor-controlled capacitive ignition system for internal combustion engines, in particular lawn mowers, chain saws end the like, in which the direction of rotation of the engine is sensed by means of a sensor counting the number of positive and/or negative half periods appearing in a charging winding in the magnetic circuit of the ignition system when the winding is influenced by a passing magnetic field generating member mounted on the flywheel of the engine.
The device described in the publication and used for sensing of the direction of rotation of the engine presupposes that the ignition system includes a microprocessor which involves additional cost for the product of a magnitude which often makes it desirable to find a simpler solution to the problem.